Confocal, optical microscopes are not capable of obtaining a transverse resolution with a definition less than approximately a half wavelength of light due to the Rayleigh or Abbe limit. If a pinhole could be brought very close to the object and illuminated from the back, then the pinhole could be made very small and the definition would be determined essentially by the size of the pinhole. Such a device has been demonstrated by Pohl at IBM, Zurich, and by others. (See D.W. Pohl, W. Denk and M. Lanz, Appl. Phys. Lett. 44 652 (1984); E. Betzid, M. Isaacson and A. Lewis, "Collection Mode Near-Field Scanning Optical Microscopy," Appl. Phys. Lett. 51 2088-2090 (1987)) Pohl used a thin drawn down glass rod, coated with a thin metal film; the glass rod was pushed against an object to abrade a small pinhole in the film. Light was injected into the glass rod and passed through the pinhole. The first microscope was a transmission microscope; later, there was a demonstration of a reflection microscope to receive or transmit the light from a hollow cone outside the thin rod. The advantage of this microscope is its high definition. However, the light level is very low and mechanical scanning is required to view an area.
Scanning tunneling microscopes using mechanical scans to obtain definitions far smaller than an optical wavelength are well-known. Mechanical scanning is itself no longer the major problem, although it is slow and necessitates using a video display of the image. One problems associated with mechanical scanning systems of this nature is the difficulty of keeping the pinhole at the right height from the object. If it is too close, the pinhole itself is damaged and if it is too far away, the definition suffers. In scanning tunneling microscopes, the spacing is controlled by using the fact that the tunneling current varies exponentially at a very rapid rate with distance from the object. Thus it is possible to make a feedback system which maintains the height constant. In a near-field optical microscope, normally the variation with distance is not as rapid, so the negative feedback systems are not as accurate. Another difficulty with this kind of optical microscope is that the amount of light passing through the pinhole is, by necessity, small. Furthermore, in a reflection near field microscope, the light reflected from the object through the pinhole is small in intensity compared to the reflections from the surrounding medium.